An aluminum nitride sintered product has been conventionally produced by such a method wherein a mixture of aluminum nitride powder (having a size of 1 .mu.m or more) with a sintering auxiliary is molded to a desired shape, and the molded article is heated in a non-oxidizing gas, such as nitrogen or argon, of an atmospheric pressure to a high temperature of the order of 1600.degree. to 1850.degree. C. to effect sintering.
As a special method, in J. Ceram. Assoc. Japan, 73 (2) 1965, pages 34-38, Shigetomo MATSUO et al. disclose production of an AlN sintered article by heating a molded mixed powder of AlN and Al in an nitrogen atmosphere. According to MATSUO et al., AlN powder of an average size of 5.67 .mu.m is mixed with Al powder of an average size of 3.27 .mu.m, the proportion of the Al powder being 20% by weight or less; and the mixed powder is molded under pressure and placed in a heating furnace, where nitrogen gas is caused to flow at a flow rate of 0.5 liter/minute to maintain a pressure of one atmosphere and the furnace temperature is raised at a heating rate of 300.degree. C./hour to 1400.degree. C. to produce an AlN sintered article. The Al powder reacts with the nitrogen gas to form AlN, and eventually a uniform AlN sintered article is produced. It is said that in an example wherein the mixed powder contained 20% by weight of Al powder, a heating temperature of at least 700.degree. C. was necessary for all of the Al powder to react with the nitrogen gas. Examples wherein the mixed powder contained 15, 10, 5 and 0% by weight of Al powder, respectively, are also shown. Furthermore, effects of the sintering temperature (furnace temperature) on the density and flexual strength of the sintered product are graphically shown (FIGS. 1 and 2). It is said that the nitridation of Al powder slowly proceeds, irrespective of the tested mixing ratio of Al powder, when the sintering temperature is raised from about 500.degree. C. to about 700.degree. C., and this is substantiated by the results shown in FIGS. 1 and 2. In other words, it is not shown that the density or flexual strength of the sintered product shows any peak value at a certain sintering temperature.